Shunt DC Generator

How accurate a value do you require?

You can probably estimate the excitation power to be about 10% of the generator maximum outout power.

Are you at the limit of what the generator will put out for the load? Where would you get field excitation current? You don't gain anything except the ability to vary the field excitation. If you increase it above what it is now, the power to drive the generator to the speed at which you get 150 V will be higher with associated increased generator heat, meaning that there's a limit before thermal breakdown of conductor insulation.

I would have thought with a higher external field voltage the speed of the generator will be less to produce 150 volts, loading at 150 volts will be the same but there is a risk of burning out shunt field wire if excessive field voltage is supplied.

Presumably the field coils are supplied via a voltage regulator which has to sense the output and supply whatever correction is required this would not be much good on an external supply without some modifications.

The slight amount of extra power available say 1.5 amp would hardly be worth the effort to modify the system

I would have thought with a higher external field voltage the speed of the generator will be less to produce 150 volts . . .

You are correct -- I apparently had a "senior moment."

Increasing the field current (by way of increasing voltage) increases the magnetic flux, produces more power at same speed, etc. Most generators of this type are designed to run at a fixed speed, and either the DC resistance or AC impedance is the only regulation of the field. New generators are fancier -- Honda makes several.

This and the previous thread, "DC motor" sound very very much like homework questions.

Do you think that a saving of 150V * 1.5A = 225W Approx are worth to provide as much power to Field windings from External Exciter running on the same-generator or else. No saving in Energy but adding problems of regulation + an extra cost.

Guest,

Did you get the answer you were looking for?

I'm curious as to why you wanted to know this? A separate excitation source is generally only used for large AC generators where it is necessary to adjust field voltage to control import and export of VARS.

NO I DID NOT!!!!!!!!!! I AM DOING A LITTLE EXPERIMENT - SO STILL YOUR HELP WILL BE APPRECIATED.

Your stated max load current was 15 amp, now it is not supplying 1.5 amp to field coils you should have 16.5 amp available max. This calculated on your originall information , If your field current is different it should have been stated and at what voltage assuming regulation is required to produce 150 volts output.

If you are experimenting stick your meters on to the generator and measure the values then you wont need any more answers from this forum.

I have to correct myself. I said that separate excitation is only used with large AC generators but I think all vehicle generators and alternators use an external excitation source (battery) and voltage regulator.

You can use your car alternator to experiment with. Connect a DC ammeter in series with the field winding and another in the battery circuit, rev the engine up to around 2000 RPM and then vary the loading by turning on the headlights, A/C or heater fan and any other electrical accessories the vehicle may have. Observe the ammeter readings as you increase and decrease the load on the alternator.

You can use your car alternator to experiment with. Connect a DC ammeter in series with the field winding and another in the battery circuit, rev the engine up to around 2000 RPM and then vary the loading by turning on the headlights, A/C or heater fan and any other electrical accessories the vehicle may have. Observe the ammeter readings as you increase and decrease the load on the alternator.

This is a great suggestion, although it is now very difficult to find an alternator with an external field terminal. Typically, the diodes and regulator are internal, so that the alternator only has one lead coming out (the frame mounting serving as the other). Not as much fun for experimenting as in the old days.

You are right Ken!

I guess my age was showing when I provided that response.

I guess my age was showing when I provided that response.

I find mine is showing almost every day.